1. Field of the Invention
The present invention relates to a fixing-unit roller such as a pressing roller used in a fixing unit in an electrophotographic copier, a printer, a facsimile, or the like to fix a not-yet-fixed toner image, and a fixing unit and an image forming apparatus including the fixing-unit roller.
2. Description of the Related Art
As fixing units used in various image forming apparatuses such as copiers, printers, facsimiles, and multifunction peripherals having functions of these machines, there have been known fixing units that include a thin fixing belt made up of a metal substrate and an elastic rubber layer, for example. Employing a thin fixing belt that is reduced in thermal capacity leads to considerable reduction in energy necessary to heat the fixing belt, thereby making it possible to reduce warm-up time (time for heating from a room temperature to a predetermined printable temperature (reload temperature) at power-on or a like situation) and first print time (time from receipt of a print request to when discharge of a sheet on which, after preparation for printing, printing is performed is finished).
FIG. 2 illustrates an example of a fixing unit. A fixing unit 40 includes a fixing belt 3 which is a rotatable fixing rotating body, a pressing roller 4 which is a opposed rotating body arranged to be opposed to the fixing belt 3 and to be rotatable, a halogen heater 23 which is a heating source that heats the fixing belt 3, a nip forming member 24 arranged inside the fixing belt 3, a stay 25 which is a support member that supports the nip forming member 24, and a reflector 26 that reflects light emitted from the halogen heater 23, toward the fixing belt 3. The fixing belt 3 includes a thin, flexible endless belt member (that may be a film).
FIG. 4 is a schematic cross-sectional view of the pressing roller 4. In this example, an elastic layer 42 of a porous material and a releasing layer 43 of polytetrafluoroethylene are laminated in this order on an outer surface of a cylindrical metal core bar 41.
According to a technique disclosed in Japanese Patent No. 4795715, an elastic layer of such a pressing roller as the pressing roller 4 is formed from a low hardness sponge having obtained by foaming silicone rubber. According to Japanese Patent No. 4795715, using such a low hardness sponge obtained by foaming silicone rubber allows not only reduction in warm-up time but also providing a sufficiently wide nip width while satisfying generally-required compactness, thereby allowing favorable image fixation. However, although this technique can reduce warm-up time, the technique is disadvantageous in that the sponge layer has low endurance because an external force applied to the sponge layer damages the sponge layer in a short period. Furthermore, hardness of the sponge layer decreases sharply, a nip pressure drops in a short period, making it difficult to maintain a predetermined nip pressure. This results in insufficient image fixation.
Accordingly, to prolong usable life of such a low hardness sponge, it is necessary to impose restrictions such as low pressure and low load by employing a pressure-releasing mechanism for the roller or the like. As a result, a usable range of the roller becomes disadvantageously narrow.
An elastic foamed heat insulation layer having open cells and formed from a water-emulsion silicone rubber composition using water as a dispersant has been proposed. The elastic foamed heat insulation layer can be obtained using what is called as a water foaming silicone method. According to this method, minute cells having open-cell structure are produced. Accordingly, an increase in outside diameter of a roller due to thermal expansion and/or a decrease in hardness resulting from breakage of cells during heating can be prevented, and therefore endurance can be increased.
However, still further increase in endurance is desired in practical use.
In view of the above, there is a need to provide a fixing-unit roller capable of providing a wide nip width even when the roller has a small diameter and reducing warm-up time while being highly durable, and a fixing unit including such a roller.